(1) Field of the Invention
The present invention relates to a displacement control valve mechanism of a variable displacement compressor used in an air conditioner for an automobile or the like and a variable displacement compressor using such a mechanism.
(2) Prior Art
Conventionally, variable displacement compressors have been used in air conditioners for automobiles and such a compressor is disclosed in Japanese Patent Publication (JP-B) No. 4-74549. In this publication, the variable displacement compressor is disclosed which has its outer frame formed by a cylinder block provided with a plurality of cylinder bores, a front housing mounted on one end of the cylinder block, and a rear housing mounted on the other end of the cylinder block while encasing a valve plate device therein.
A crank chamber is defined between one end of the front housing and one end of the cylinder block. A drive shaft is disposed in the crank chamber. A swash plate mechanism is disposed around the drive shaft. The swash plate mechanism includes a tilting plate, a drive plate, and a rotor. In the swash plate mechanism, the drive plate carries out a tilting movement. The drive plate comes into contact with the tilting plate. The rotor is connected to the drive plate for driving the drive plate. A discharge chamber and a suction chamber are defined in the rear housing.
Inside the bores of the cylinder block, pistons are disposed which are connected to the periphery of one end of the tilting plate through piston rods.
In such a conventional variable displacement compressor, with the adjustment of the pressure in the crank chamber, the inclination angle of the tilting plate is changed so as to change the compressive volume. The pressure control in this crank chamber is carried out by a displacement control valve mechanism mounted in the bottom wall of the other end of the rear housing.
This conventional type of displacement control valve mechanism is called a pressure control valve of an inner control type. With respect to this control valve mechanism, a pressure sensing space and a valve chamber portion are defined in the casing and a pressure sensing member is disposed in the pressure sensing space. The pressure sensing space and a suction chamber are communicated with each other. The valve chamber portion is communicated with discharge chamber by way of a communication hole which is communicated with the discharge chamber and a communication chamber which ensues the communication hole. A through hole is communicated with the crank chamber.
In such a conventional control valve, the pressure sensing member accommodated in the pressure sensing space senses a pressure of the suction chamber and vertically moves a valve body of the valve chamber portion in response to the pressure of the suction chamber. Furthermore, the pressure sensing member adjusts an opened degree of a first passage extending from the discharge chamber to the crank chamber.
In such a conventional displacement control valve mechanism, the pressure control characteristics in the suction chamber of the displacement control valve mechanism is designed such that the pressure in the suction chamber (hereinafter called simply as suction chamber pressure) is changed in response to the pressure in the discharge chamber (hereinafter called simply as discharge chamber pressure).
However, the control characteristics of the suction chamber pressure is determined to have the optimum characteristics in a condition where the compressor is mounted on a vehicle. Accordingly, the optimum characteristics varies depending on vehicles so that the displacement control valve mechanism which has several kinds of control characteristics of the suction chamber pressure becomes necessary.
In one method, the characteristic, i.e., the characteristic curve representing the realizable relationship between the suction chamber pressure and the discharge chamber pressure, is changed by shifting it up and down by adjusting the amount of displacement of a spring inside a bellows portion of the pressure sensing member with an adjustment screw.
Alternately, in another method, the characteristics is optimized by changing the amount of variation of pressure in the suction chamber relative to the pressure in the discharge chamber. In this method, the conventional structure necessitates the change of the seal area of a valve body made of a ball valve or the effective area of the bellows portion.
However, the change or alteration of the effective area of bellows is not preferable in terms of designing since it requires a large design change of the displacement control valve mechanism. On the other hand, in case the seal area of the valve body made of the ball valve is changed, an inflow rate of the discharge gas into the crank chamber is changed so that the rise characteristics of the pressure in the crank chamber is changed. Hence, a problem that the pressure control of the suction chamber becomes unstable, arises.